Since their introduction, hand held or "cordless" telephones have enjoyed a substantial popularity. Such telephones include a hand held unit or "handset" which looks much like a telephone but includes means for establishing a duplex radio link over a channel having two frequencies with a base unit which is hard wired to the telephone line. In use, communication is established between the base unit and the handset either upon initiation of a call by the handset user, or an incoming telephone call received over the telephone line. To the user, the handset must function as much as a standard telephone as possible, while providing mobility. The handset typically includes rechargeable batteries such as nicads which are recharged by placing the handset in the base unit for convenient storage and electrical connection to a battery charging power supply.
Given the fixed amount of radio spectrum, only a relatively small portion has been allocated to cordless telephones. Since cordless telephones are to a large degree unregulated and, unlike mobile phones or cellular telephones significantly less sophisticated, they inherently have certain limitations. Cordless telephones operate on a selected band of a relatively few channels. The selection is made somewhat permanently, i.e., by hardwiring or by the setting of a switch which may or may not be readily accessible by the user. Typically, the base unit and the handset are set to the same channel once and the channel setting is not changed thereafter. In fact, it is likely that few end users are actually aware of the ability to change channels in those cordless telephones which have this capability.
Regardless of whether a given cordless telephone has the ability to select one of a plurality of channels for use, the likelihood is high that a particular location may be undesirably within the communication range of more than one cordless telephone user on the same channel. Given the relative density of urban and suburban areas, and the desirability of having a cordless phone with sufficient communication range to allow its use within a reasonable proximity of a dwelling, a number of cordless telephone users in a given neighborhood may find themselves causing radio frequency interference with their neighbor's cordless telephones. Given also the relatively small number of channels used for cordless telephones, it is also relatively simple for a person to select a channel on his cordless telephone which is the same as the channel of another user. While this may allow an interloper to eave drop on another's conversation, perhaps more dangerous is the potential for the interloper to gain access to a neighbor's telephone line, allowing the interloper to answer or place calls on the neighbor's telephone line.
For these reasons, it is common to employ a security code in cordless telephone systems. Thus, in addition to selecting one of a relatively small number of channels, a cordless telephone user is able to establish a security code, much like a security code is established by persons utilizing remote control garage door openers on a common frequency. A simple security code in the prior art is the sub-audible tone which is encoded by a transmitter and detected by a receiver. If the appropriate one of the standard sub-audible tones is detected, communication is established. If not, communication is not established. A sub-audible tone is lower in frequency than tones which can be heard by the user, so there is no annoying sound heard by the user when the sub-audible tone is used.
A more sophisticated security system utilizes an N bit digital word which is typically transmitted at the beginning of the use of the cordless phone. Utilizing an N bit security code, 2.sup. N possible security codes are provided. The security code can either be factory preset so that the cordless handset and base unit include identical security codes, or may be set by the user, for example via DIP switches located on the handset and the base unit. This technique guards, at least to a certain extent, against the possibility of unintentional or unauthorized access to a user's telephone line.
U.S. Pat. No. 4,593,155 describes a cordless telephone system in which the handset is capable of learning the preset security code stored in its associated base unit. U.S. Pat. No. 4,731,813 describes such a cordless telephone system in which communication between the base unit and the handset of the security code information is accomplished by modulating the charging current supplied to the handset from the base unit.
A disadvantage of prior art security systems are that there are a relatively small number of sub-audible tones which can be sequentially tested by a would be intruder. Similarly, while there may be a greater number of digital security codes available, depending upon the number of bits N in the security word, given enough time and patience, a would-be intruder can determine the security code of a neighbor's cordless telephone system merely by trial and error.
Accordingly, there remains the need for providing greater security against would be intruders in cordless telephone systems.